Disable bitcoin_hashes default features

Currently we use default features for the `bitcoin_hashes` dependency,
doing so breaks the `no-std` feature in `rust-bitcoin` because `std` is
part of `bitcoin_hashes` default feature set.

Disable `bitcoin_hashes` default features, no changes to `rust-bitcoin`
are require after this change since we manually turn on `std` and
`alloc` as part of the `std`/`no-std` features of `rust-bitcoin`.

For other users of `rust-secp256k1` this is a breaking change but is
unlikely to cause too much bother because `std` is so commonly used.

Author: tcharding

Cargo.toml

@@ -38,7 +38,7 @@ global-context-less-secure = []
 [dependencies]
 secp256k1-sys = { version = "0.4.2", default-features = false, path = "./secp256k1-sys" }
 
-bitcoin_hashes = { version = "0.10", optional = true }
+bitcoin_hashes = { version = "0.10", default-features = false, optional = true }
 rand = { version = "0.6", default-features = false, optional = true }
 serde = { version = "1.0", default-features = false, optional = true }